In recent years, atmospheric pollution has become a source of major concern to government, industry and individuals alike. Even minute quantities of noxious gases and other chemical irritants present in the air can pose serious health and safety problems for persons exposed to the irritants. These problems may be area-wide or even regional in nature, as is often the case with automobile smog or heavy industrial pollution, or they may be confined to particular working environments such as chemical factories, oil, refineries or mining operations.
With regard to particular working environments, the Occupational Safety and Health Administration (OSHA) and other U.S. Governmental agencies have developed standards which set maximum allowable levels for exposure to toxic gases. Efforts to meet these standards, and indeed all attempts at controlling pollution from whatever source, quite obviously depend upon the ability to measure low pollutant levels accurately. Accordingly, numerous types of instruments for determining the presence and quantity of atmospheric pollutants have heretofore been proposed, manufactured and made available to the public.
Instruments for measuring the composition of the atmosphere utilize a wide variety of analytical techniques, including electrochemical techniques, gas chromatography, chemiluminescence, colorimetry and spectrophotometry. Electrochemical methods in particular have been the subject of extensive activity in the field of atmospheric analysis. The advent of modern integrated circuitry, coupled with the concomitant proliferation or reliable and inexpensive electronic components, has spurred the development of a wide variety of simple, compact measuring instruments with low power requirements. Recent examples of gas monitoring apparatus based on electrochemical methods can be found in U.S. Pat. No. 3,314,863, issued to Hersch et al on Apr. 18, 1967, and U.S. Pat. No. 3,821,090, issued to Topol et al on June 28, 1974.
Electrochemical cells for use in gas monitoring devices usually comprise three electrode potentiostats, which potentiostats include an indicating electrode, a reference electrode, and a counter or auxiliary electrode all immersed in a suitable electrolyte. Depending on the composition of the cell components and the value of the voltage applied across the indicating and counter electrodes, the gas being monitored is either electroreduced or electro-oxidized to produce an electrode current indicative of the amount of gas present.
As may be surmised, the effectiveness of any given electrochemical monitoring cell in detecting a specific type of gas is contingent upon obtaining the proper arrangement of cell components, and materials for forming the components must be carefully chosen in accordance with the electrochemical reaction desired. U.S. Pat. No. 4,152,233, issued to Chand on May 1, 1979, provides a representative list of compatible sensing electrodes, reference electrodes, counter-electrodes and electrolytes for each of a number of detectable gases. Modification of the structural features of these components can lead to further improvements in overall cell performance. For example, U.S. Pat. No. 4,057,478, issued to Bruckenstein et al on Nov. 8, 1977, teaches a means for constructing an electrode supporting substrate out of porous Teflon sheeting in order to provide increased stability during electrochemical cell operations. Additional prior art teachings have enabled the construction of gas monitors with improved accuracy and response time.
In spite of recent advances in the electrochemical cell art, many problems associated with the use of electrochemical cells to detect certain gases remain. One of the most serious of these problems results from the interfering presence of other species of gas in the sample of air being monitored. Devices of the type disclosed in the above-mentioned Hersch et al patent, which rely upon the oxidization of halogens in response to the passage of gaseous pollutants over a halide electrolyte, are susceptible to interference from chloride, bromine and organic sulfur compounds. Similarly, anodic oxidiation of nitrogen dioxide to provide an electrochemical current as disclosed in Topol et al, also mentioned above, is subject to serious interference from both sulfur dioxide and carbon monoxide. It is thus often necessary to employ multiple filtration systems for removing interferents from a sample of gas prior to passage of the gas through the electrochemical monitoring cell. U.S. Pat. No. 3,677,708, issued to Harman III, et al on July 18, 1972, illustrates a multiple filtration system particularly useful with the Hersch et al device. These systems, however, inevitably add to the overall complexity and bulk of the monitor, and render the production of portable monitoring equipment more difficult.
Another obstacle to construction of truly practical atmospheric pollutant monitors arises out of the desire to measure more than one species of gas at a time. As disclosed in U.S. Pat. No. 3,622,487, issued to Chand et al on Nov. 23, 1971, and U.S. Pat. No. 3,763,025, issued to Chand on Oct. 2, 1973, prior art devices for detecting the combined presence of two oxidizable gases such as NO and NO.sub.2 are known. Although many such devices are unable to distinguish between various atmospheric constituents and instead simply measure a cumulative pollutant total, other monitors do separate out and quantitatively indicate the individual noxious components present in a sample of air. The device disclosed in the above-mentioned Hersch et al patent, for instance, may be adapted to specifically determine ambient concentrations of both NO and NO.sub.2. The electrochemical cell utilized by Hersch et al is primarily designed to measure NO.sub.2, and any NO present in a gas sample will not interfere with the NO.sub.2 analysis. On the other hand, the detection of NO in the electrochemical cell can only proceed after first routing the sample through a filter which removes all of the NO.sub.2 initially present in the sample. Subsequently, the NO is chemically oxidized to NO.sub.2 and passed through the cell to provide an indirect measurement of the amount of NO present in the sample. While Hersch et al accordingly provide a means for quantitatively isolating two oxidizable components of a sample of air, the requirement of a filtration step between the two component measurements significantly complicates the operation of the NO/NO.sub.2 monitor. Furthermore, even though Hersch et al use the same electrochemical cell structure to detect both NO and NO.sub.2, there is nothing in the Hersch et al patent which suggests an arrangement suitable for simultaneous, as contrasted with sequential, monitoring of NO and NO.sub.2. Consequently, in situations involving rapid fluctuations in the amount of noxious pollutants present in the atmosphere, the Hersch et al device would prove of little value in obtaining truly representative measurements of pollutant concentrations.
Prior art devices capable of simultaneously detecting multiple gaseous pollutants in an atmospheric sample are disclosed in U.S. Pat. No. 3,776,832, issued to Oswin et al on Dec. 4, 1973; U.S. Pat. No. 3,909,384, issued to Jasinski et al on Sept. 30, 1975; U.S. Pat. No. 4,001,103, issued to Blurton et al on Jan. 4, 1977; and U.S. Pat. No. 4,007,096, issued to Jasinski et al on Feb. 8, 1977. All of these prior art devices employ a separate electrochemical cell for each gas monitored, and the corresponding electronic circuitry must be expanded to include the electrical voltage supply, potential sequencing and detection functions necessary to the operation of each separate cell. In addition, the Blurton et al device requires an external filtration system for isolating the various noxious components of the air sample to be analyzed. Thus, regardless of their increased sensitivity to rapid changes in the atmosphere, none of the latter pollutant monitors is particularly suited to inexpensive manufacture in the form of a compact portable monitoring unit.